1. Field of the Invention
The present invention concerns a device for protecting a direct current electrical power supply from disturbances caused by connecting to it or disconnecting from it an electronic system.
2. Description of the Related Art
Complex electronic equipment such as telecommunication equipment usually comprises electronic systems such as printed circuit boards equipped with electronic components which plug into subracks mounted in racks or cabinets.
The electrical power required by the electronic components carried by such boards is usually provided by a direct current electrical power supply unit which is shared by the various boards in the same rack.
The supply voltage input circuit of each board usually includes one or more filters to block from the electronic circuits on the board any electrical interference on the supply voltage and to prevent retransmission to the supply of any electrical interference generated when the board is in operation.
A typical filter circuit is shown in FIG. 1; this is an LC filter circuit comprising an inductor L1 and a capacitor C2, the DC supply voltage V being applied to the terminals of the combination L1-C2 and the output voltage being taken across the capacitor C2.
The effectiveness of the filtering action of the circuit is proportional to the values of the inductor L1 and the capacitor C2. However, the values of these components are optimized to allow for other criteria including overall size and cost.
The presence of filters on the board increases the level of electrical disturbances occurring when the boards are plugged in or unplugged, to a degree that is proportional to the current drawn by the board, which is increasingly high because of the increasing trend towards a high concentration of circuits on the board.
When a board having a filter circuit of this kind is connected to the supply, the capacitor C2 is very quickly charged through the inductor L1 which draws a high current from the supply.
The supply is generally a DC/DC converter (for example a 48 V/5 V converter in the case of telecommunication equipment). Although the converter includes a voltage regulator, the time for regulation to become effective is generally long in comparison with the time to charge the capacitor C2 and the result is a temporary drop in the supply voltage while the capacitor is charging, which can disturb the operation of some circuits operating at this time on other boards.
When a board including a filter circuit of this kind is disconnected, the sudden interruption of the current in the inductor belt L1 produces a voltage surge that can in practice have an amplitude of several tens of volts, which causes arcing at the contact of the board connector when the board is disconnected.
During this very short interval (in practice a few microseconds) the voltage surge is transmitted towards the supply and can disturb the operation of some circuits as it gives rise to an electrostatic disturbance which generates electromagnetic disturbance.
One solution to the problems described above occurring when the board is connected is to increase the value of the inductor L1. The time to charge the capacitor C2 would then be increased, enabling the voltage regulator of the DC/DC converter to correct the voltage drop, but the voltage surge on unplugging the board due to the inductor L1 would be of even greater amplitude.
This solution would also lead to an increase in overall size and cost.
Another solution would be to reduce the value of the capacitor C2 or to connect a resistor in series with the capacitor, increasing the impedance presented to the supply and reducing the current drawn from the supply when the board is connected.
This solution would reduce the performance of the filter circuit on the board, however.
An object of the present invention is to avoid these drawbacks, that is to say to avoid the generation of the disturbances described above affecting the common power supply of the boards when boards are connected or disconnected, without compromising the effectiveness of filter circuits on the boards.